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Nickel disulfide-graphene nanosheets composites with improved electrochemical performance for sodium ion battery Tianshi Wang, Pu Hu, Chuanjian Zhang, Huiping Du, Zhonghua Zhang, Xiaogang Wang, Shougang Chen, Junwei Xiong, and Guanglei Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00179 • Publication Date (Web): 08 Mar 2016 Downloaded from http://pubs.acs.org on March 16, 2016

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ACS Applied Materials & Interfaces

Nickel Disulfide-graphene Nanosheets Composites with Improved Electrochemical Performance for Sodium Ion Battery Tianshi Wang, a,b† Pu Hu, b† Chuanjian Zhang, b Huiping Du, b Zhonghua Zhang, b Xiaogang Wang, b Shougang Chen, a* Junwei Xiong,c* Guanglei Cui.b* a

Institute of Materials Science and Engineering, Ocean University of China, Qingdao

266100, Shandong Province, P. R. China b

Qingdao Industrial Energy Storage Research Institute, Qingdao Institute of

Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China c

Key Laboratory for Liquid-Solid Structural Evolution & Processing of Materials

(Ministry of Education), Shandong University, Jinan 250061, P. R. China † These authors contributed equally to this work.

Abstract: Nickel disulfide-graphene nanosheets (NiS2-GNS) composites were successfully synthesized via a simple and mild hydrothermal method. It was revealed by SEM and TEM images that the spherical NiS2 nanoparticles with a diameter of 200-300 nm were uniformly dispersed on graphene nanosheets. Na electrochemical storage properties including cycling performance and high-rate capability of NiS2-GNS composites were investigated, demonstrating a superior reversible capacity of 407 mAh g-1 with the capacity retention of 77 % over 200 cycles at a current density of 0.1 C. Furthermore, even at a large current density of 2 C, a high capacity of 168 mAh g-1 can still remain, which is much higher than that of pristine NiS2

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materials. The enhancement in electrochemical properties might be attributed to the synergetic effect endowed by high conductivity of graphene and novel structure of the electrode material. Combined with the advantages of low cost and environmental benignity, NiS2-GNS composite would be a potential anode material for sodium ion batteries. Keywords: graphene nanosheets, transition metal disulfides, nickel disulfide, anode materials, sodium ion battery

INTRODUCTION Although lithium ion batteries (LIBs) have been widely applied in portable electronic devices in the past decades, the world’s limited lithium reserves restrict the future mass application of LIBs in large-scale energy storage devices 1. Sodium (Na)-ion batteries (SIBs), possessing similar electrochemical principle and manufacturing process with LIBs, have raised increasing attention as a potential alternative for LIBs in virtue of elemental abundance, accessibility, low toxicity, proper ion radius (ca. 1.06 Å for Na vs. ca. 0.76 Å for Li) and potential (Na, -2.71V; Li, -3.04V) of sodium. In the past years, many cathode materials of Na-ion battery based on layered metal oxides

2-3

, polyanionic compounds

4-5

have been explored, exhibiting excellent

comparable properties to those of Li counterpart. However, the lack of suitable anode materials leads to inferior electrochemical performances of SIBs to those of Li counterpart. Carbonaceous materials are a series of typical anode materials in SIBs exhibiting good cycle stability and low cost

6-7

whereas delivering low specific


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capacity

8

and low charge-discharge plateau close to deposition potential of sodium,

which may lead to the formation of sodium dendrites 9. It is reported that alloy metal materials and transition metal oxides (TMO) exhibit high capacity, however, severe capacity fading limits their application due to the huge volume expansion during the charge-discharge process and their poor intrinsic conductivity 10-15. Compared to TMO, transition metal sulfides (TMS) have higher gravimetric energy density and superior Na storage properties, due to their excellent electrical conductivity, unique electronic structure and other practical performances

16-19

Numerous reports on TMS as anode materials for SIBs have been published

20-23

. .

Among them, Ni-based sulfides including Ni3S2 and NiS have been explored, which reported to be a conversion reaction mechanism during the charge-discharge process (MS2 + 4Na = M + 2Na2S) 24-25. Nickel disulfide (NiS2), as a stable compound of the nickel sulfides family, has a higher theoretical capacity (807 mA h g-1) in SIBs than those corresponding nickel sulfides members (444 mA h g-1 for Ni3S2 and 533 mA h g-1 for NiS), according to the conversion reaction (NiS2 + 4Na = Ni + 2Na2S). As far as the best of our knowledge, no study about NiS2 have been explored as anode material in SIBs yet. Like other TMS anodes, NiS2 would suffer from volume expansion and structure failure during cycling as well. To overcome these drawbacks, various strategies such as compositing, reducing crystallite size and nanostructuring are often applied

26-28

.

Among them, graphene is considered to be an ideal strategy for buffering volume expansion during charge-discharge process for its proper layer distance and flexibility,



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which is beneficial to prevent the aggregation of the active material particles. Meanwhile, excellent electrical conductivity endows it a reasonable electronic transport pathway to compensate the limited electrical conductivity of active materials. Therefore, decorating NiS2 nanoparticles with graphene is expected to be an effective way to obtain an improved electrochemical performance of the electrode materials. Herein, nanostructured nickel disulfide-graphene nanosheets composites (NiS2-GNS) were synthesized via a facile one-step hydrothermal method. The incorporation of graphene nanosheets buffers volume change and enhances electrical conductivity of pristine NiS2, allowing fast Na ions and electrons transport and simultaneously ensuring structural integrity. As a result, the NiS2-GNS would deliver superior electrochemical properties including high rate capability and good cycling stability, making them potential anode materials for rechargeable SIBs.

EXPERIMENTAL SECTION All the purchased reagents were used without any further treatments. Graphene oxide (GO) was prepared through the modified Hummer’s method

29

. Firstly, 3 mL of the

as-prepared GO suspension (4 mg mL-1) was distributed in 30 mL ethylene glycol with magnetic stirring to form uniform solution. 1.2 mmol thioacetamide (0.09 g) was added to ethylenediamine aqueous solution (12 mM, 30 mL) with 0.5 h stirring to form a transparent solution and subsequently drop it into the GO dispersion. The mixed solution was agitated for another 0.5 h. Then 10 mL NiSO4 aqueous solution (0.6 mmol) was added into the mixed solution by drops with vigorous agitation for


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one hour. The above solution was then transferred into a 100 mL autoclave and kept in an electric oven for 12 h at 150 °C. After cooling, the black precipitate was centrifugated and washed with de-ion water and ethanol for several times and then subjected to freeze drying, fluffy sponge-like NiS2-GNS nanocomposite was obtained. For comparison, pristine NiS2 nanoparticles and reduced graphene oxide (RGO) were prepared via the same synthesis method. For electrochemical measurements, the working electrode was prepared by mixing active material, carbon black and polymer binder (Polytetrafluoroethylene, PTFE) in a weight ratio of 7:2:1. The mixture was then pressed onto stainless steel mesh and dried at 60 °C in vacuum for 12 h. The test cells were assembled in an argon-filled glovebox. For half-cell studies, the counter/reference electrode was a sodium metal foil and a glass fiber GF/D (Whatman) filter was used as the separator. A 1 M solution of NaClO4 in EC/DMC (1:1 by volume) with 5% FEC was prepared and used as the electrolyte. Galvanostatic charge-discharge experiments at various C-rates (1 C = 807 mA g-1) were tested in a LAND battery testing system. Cyclic voltammograms (CVs) were performed using a CHI 440A instrument (CHI Instrument Inc.) at a scanning rate of 0.1 mV s-1. All the tests were performed at the room temperature. Electrochemical impedance spectroscopy (EIS) was tested via an electrochemical workstation (ZAHNER-Elektrik GmbH & Co. KG, Germany) in the frequency range from 10 mHz to 100 kHz with an amplitude of 5 mV. Powder X-ray diffraction (XRD) patterns were collected with a Bruker-AXS Micro-diffractometer (D8 Advance) using Cu Kα radiation (λ = 1.5406 Å). Scanning



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electron microscope (SEM) images and energy dispersive spectra (EDS) were obtained using a Hitachi S-4800 field-emission electron microscope. Transmission electron microscope (TEM) observations were gained on a Hitachi HT-7700 electron microscope operating at an accelerating voltage of 100 kV. High resolution transmission

electron

microscope

(HRTEM)

images

were

taken

on

a

JEOLJEM-2010F microscope (JEOL, Japan) at an acceleration voltage of 200 kV. Raman spectra were collected at room temperature by Thermo Scientific DXRxi system with a laser wavelength of 532 nm.

RESULTS AND DISCUSSION

Figure 1. (a) XRD patterns of NiS2-GNS and pristine NiS2; (b) Raman spectrums of the as-prepared NiS2-GNS, NiS2 and GO. The structure and composition of the obtained materials were characterized in details. The typical diffraction peaks of pristine NiS2 and NiS2-GNS from XRD patterns (Figure 1a) at 31.5°, 45.1° and 53.5° are clearly observed, which correspond to the (200), (220) and (311) lattice planes of cubic-phase NiS2 with the space group Pa3 (JCPDF#65-3325), respectively. The sharp peak intensity demonstrates their excellent crystallinity. Compared with the pattern of pristine NiS2, an extra broad peak appeared


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at around 24°, which is located in the same position but much more flat than the reported peak of reduced graphene oxide 30. The change in this peak may be ascribed to the insertion of active materials in the regular stacking of graphene nanosheets according to the previous reports

31-32

, confirming the good incorporation between

NiS2 nanoparticles and graphene nanosheets. Table S1 summarized the crystallite diameters of NiS2 in the composite and pristine NiS2 calculated via the Scherrer’s equation using the detailed information of respective XRD peaks at around 31.6°, 35.5° and 38.9°. The average crystallite sizes of NiS2-GNS and NiS2 are estimated to be 23.8 ± 0.5 nm and 37.5 ± 0.5 nm, respectively. The average size of pristine NiS2 particles is approximately 50% larger than that of NiS2-GNS, indicating that the incorporation of graphene nanosheets can be effective to prevent the growth of crystallite sizes during the hydrothermal process. In order to investigate the reducing degree of graphene nanosheets used here, Raman spectroscope analysis was applied (as shown in Figure 1b). Two characteristic peaks at 1350 and 1580 cm-1 in Raman spectra of NiS2-GNS are corresponding to the D and G bands of graphene 33, respectively. The D band corresponds to the vibrations of sp3 carbon atoms in edge or structural defects; the G band is related to the vibrations of sp2 carbon atoms in graphite

34-35

. The ratio of the D and G band intensities (ID/IG) of

NiS2-GNS is 1.13, and the ID/IG of the as-prepared RGO is 1.09. The increase of ID/IG after incorporation shows that more disordered carbon appeared in NiS2-GNS, delivering a highly reducing degree along with rich defects, which benefits for improving electrical conductivity of NiS2-GNS and providing more sites for Na+



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storage

35-36

. There are two obvious peaks located at 271 and 476 cm-1, which are

consistent with Eg and Ag photons of NiS2 37, respectively. All these results indicate that after the one-step hydrothermal reaction, both pure NiS2 and reduced graphene oxides could be incorporated into the composite, which is in good accordance with the results of XRD analysis.

Figure 2. Typical SEM images of (a) NiS2-GNS composite (×5k) and (b) pristine NiS2 powder (×5k), and inset pictures were SEM images at high magnification (×100k); (c) NiS2-GNS composite at low magnification (×2k); (d) diameter distribution of the NiS2 nanoparticles loaded in the graphene sheets. The morphology of the as-prepared NiS2-GNS composite and pristine NiS2 is shown in Figure 2 by SEM imaging. Sub-microspheres of NiS2 are uniformly wrapped and distributed inside the graphene sheets, and only a few particles appear out of graphene sheets, while severe aggregation exists in the pristine NiS2 powder via the same synthetic route of NiS2-GNS, which indicates that the graphene sheets play an important role in preventing the aggregation of the NiS2 sub-microspheres during


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synthetic process. From the inset picture of the pristine NiS2 sub-microsphere at high magnification, some enlarged secondary nanoparticles can be seen on its surface, which can be confirmed by the following TEM tests. The SEM image of NiS2-GNS illustrated in Figure 2c can further confirm the uniform distribution of active materials. Figure 2d presents a histogram of the diameter distribution of NiS2 sub-microspheres, in which the sub-microspheres of 150-350 nm take the major part (83.2%) of all, indicating desired uniformity of the nanocomposite.

Figure 3. Typical TEM image of (a) NiS2-GNS at low magnification and the inset picture was the corresponding SAED pattern; (b) HRTEM image of NiS2-GNS composite Figure 3a illustrates the TEM image of NiS2-GNS composite at low magnification where similar morphology features to the SEM images shown above can be observed, where NiS2 sub-microspheres are well-organized across the graphene sheets. Those sub-microspheres are composed of secondary nanoparticles with an average size of about 30 nm, which is finely consistent with the calculated results of XRD results. Selected area electron diffraction (SAED) pattern of NiS2-GNS shows clear diffraction rings, which can be well indexed to NiS2 phase. HRTEM image of NiS2-GNS illustrated in Figure 3b shows the lattice spacing of 2.85 Å, which could be indexed to (200) lattice planes of NiS2

38

, and clear coating layers of GO with the



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lattice spacing of 3.35 Å, indexed to (002) lattice planes of graphite. The HRTEM results can further confirm that the NiS2 nanoparticles are well incorporated into reduced graphene oxide. The weight ratio of NiS2 and graphene nanosheets was further estimated by TG analysis. As shown in Figure S1, the slight weight loss before 350 °C can be attributed to the removal of absorbed water. In the temperature range of 350-400 °C, nickel sulfides went through a series of oxidization reactions along with the formation of NiS, NiSO4, and NiO39. The weight loss from 500 °C to 550 °C can be associated to the removal of oxygen-containing groups and graphene decomposition. The final stage of weight change from 650 °C to 700 °C indicates the transformation of NiS, NiSO4, and NiO, while the terminal residue was NiO. Through TGA, the calculated weight ratio of NiS2 in NiS2-GNS composites is around 50%, which is considerable for achieving the balance between capacity and electronic conductivity.

Figure 4. (a) Cycle performance with corresponding coulombic efficiency and (b) charge-discharge curves of NiS2-GNS electrode at a current density of 100 mA g-1; (c)


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rate performance of NiS2 and NiS2-GNS and (d) charge-discharge curves of NiS2-GNS at different current rates. Cycle performances of NiS2-GNS and pristine NiS2 electrodes in SIBs at the current density of 0.1 C are demonstrated in Figure 4a. It is shown that 77.0% specific capacity of the initial reversible discharge capacity (the 5th cycle, 407 mAh g-1) retained after 200 cycles for NiS2-GNS electrode, while the pristine NiS2 got a rapid capacity fading during the first few cycles, indicating that an enhanced cycle performance has been obtained after the incorporation between NiS2 and graphene. The corresponding charge-discharge curves of NiS2-GNS for the 1st, 2nd, 5th. 10th, 50th, 100th and 200th cycles are shown in Figure 4b. In this graph, two charge voltage plateaus at 1.7 V and 2.0 V and two discharge plateaus at 1.5 V and 1.0 V can be clearly observed without obvious shifts during cycling. For NiS2-GNS electrode, it suffers a big capacity loss (32%) along with a low initial coulombic efficiency (65%) in the first cycle, which may be attributed to the decomposition of electrolyte and the formation of solid electrolyte interface (SEI) layer on the surface of the NiS2-GNS electrode. In the following cycles, the discharge and charge capacities tend to stay steady at about 400 mAh g-1, while the coulombic efficiency rises to nearly 100% (the 5th cycle, 97.3%). It can be deduced that the SEI layer formed during cycling is stable, which can be beneficial for inhibiting irreversible reactions. Rate performances and their charge-discharge curves are illustrated in Figure 4c and 4d. NiS2-GNS delivers reversible capacities of 382, 375, 325, 278, 221 and 168 mAh g-1 at the current densities of 0.1, 0.2, 0.5, 1.0 and 2.0 C, respectively, and then



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recovers to 286 mAh g-1 while the current density reverses back to 0.1 C. For comparison, rate performance of pristine NiS2 at the same rates as NiS2-GNS is also displayed. The initial discharge capacity hits to 875 mAh g-1 but decreases rapidly in the following cycles and gets reversible capacities of 58, 25, 13 and 9 mAh g-1 at current densities of 0.2, 0.5, 1.0 and 2.0 C, indicating much worse rate performance of pristine NiS2 in comparison to that of NiS2-GNS. To represent rate performance of NiS2-GNS more specifically, charge-discharge curves are shown in Figure 4d. When the current density increases to 0.5 C, no obvious polarization for voltage plateaus can be observed and the reversible capacity at this current density is 85% of that at 0.1 C. Even when the current density increases to 2 C, 45% reversible capacity retains with small voltage polarization (about 0.15 V for both charge and discharge plateaus). All the results indicates that NiS2-GNS delivers much better electrochemical properties compared to pristine NiS2. This enhanced electrochemical performance can be attributed to the incorporation of graphene nanosheets that provide excellent electrical conductivity to compensate the fair intrinsic conductivity of NiS2 and the host for buffering volume change to prevent structure failure. The SEM image of the NiS2-GNS electrode after 100 cycles shown in Figure S2b is to investigate the morphology changes. Those NiS2 sub-microspheres are well enveloped with certain thin film after cycling compared to the primary electrode (Figure S2a), which is likely to be associated with the formation of SEI film. For further understanding about this change, XRD analysis is applied (shown in


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Figure S2b). After 100 cycles at the current density of 100 mA g-1, only two weak peaks can be seen in the XRD pattern of the NiS2-GNS electrode, which correspond to the (211) and (220) lattice planes of the cubic-phase NiS2 (JCPDF#65-3325), indicating the decreased crystallinity of NiS2. The XRD results are in good agreement with the SEM image of the cycled electrode, showing an amorphization trend for the NiS2-GNS composite during cycling. Previous literatures validate that isotropous expansion/contraction is more likely to happen in those amorphous phase, which benefits to relieve anisotropic mechanical stress40, 41. As a result, the excellent cycle stability can be attributed to the robust structure of the formed amorphous NiS2 along with reduced mechanical stress.

Figure 5. CV curves of (a) NiS2-GNS and (b) pristine NiS2 electrodes at a scanning rate of 0.2 mV s-1. The electrochemical processes of NiS2-GNS electrode and pristine NiS2 electrode in SIBs were further evaluated by CV technique and the corresponding curves for the first three cycles at a scanning rate of 0.2 mV s-1 were displayed in Figure 5. Two oxidation peaks at 1.70 V and 2.01 V and two reduction peaks at 0.83 V and 1.05 V can be observed in the first cycle (Figure 5a), while the former two are located at the same voltage and the latter two peaks shift to 0.95 V and 1.45 V in the second cycle.



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Both oxidation and reduction peaks of the 2nd cycle remained the same position in the third cycle, showing the desirable cycle stability. The initial reduction peak at 1.05 V with the largest current intensity indicates some irreversible reactions, delivering high irreversible capacity, which is consistent with the huge loss of current intensity of the reduction peaks in the subsequent cycle. According to the previous reports

21-23

, conversion reactions happening when

discharging in the TMS (M = Fe, Co, Ni) electrodes can be presumed as following equations ， while NaxMS2 is the intermediate product of intercalation reaction, demonstrating that it is a multi-step reaction mechanism during charge-discharge process on NiS2-GNS electrode: MS2 + xNa+ + xe- → NaxMS2

(Eq. 1)

NaxMS2 + (4-x)Na+ + (4-x)e- → M + 2Na2S

(Eq. 2)

In the second cycle, the reduction peak at 1.45 V represents the formation of NaxNiS2 as the intermediate product of intercalation reaction and the peak at 0.95 V indicates the formation of Ni and Na2S as the conversion products, while the two oxidation peaks correspond to the formation of NaxNiS2 as the conversion product between Ni and Na2S (1.70 V) and the formation of NiS2 as the product from the extraction reaction of NaxNiS2 (2.01 V), respectively

20

. Those two pairs of redox peaks were

almost overlapped in the third cycle, showing excellent reversibility of both intercalation/extraction and conversion reactions. All the results in the CV curves of NiS2-GNS are well matched with the galvanostatic charge-discharge curves above. For comparison, the CV curves of pristine NiS2 are conducted in the Figure 5b at the


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same sweep rate as well. It delivers a similar curve for the first cycle to that of NiS2-GNS with a little smaller oxidation current. Both the oxidation and reduction peaks become weaker during the following cycles, along with larger potential polarization in the cathode scanning. The gradual degradation of intensity of redox peaks indicated the poor stability of the pristine NiS2 electrode. Due to its poor electrical conductivity and aggregation, which were confirmed by the results of both SEM and TEM images, it goes through sluggish charge and ion transfer process across the pristine NiS2 electrode, leading to larger voltage polarization between the redox peaks. All these results are in accordance to the galvanostatic charge-discharge curves above.

Figure 6. Cyclic voltammetry curves of (a) NiS2-GNS at different scan rates and (b) log i -log ν plots of cathodic (~0.95 V) and anodic (~1.7 V); (c) Calculated surface desorption and bulk conversion charge currents at ~1.7 V of NiS2-GNS ; (d) Calculated surface adsorption and bulk conversion discharge currents at ~0.95 V of NiS2-GNS.



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In order to obtain further knowledge about the electrochemical process on the NiS2 electrode, cyclic voltammetry tests with different scan rates from 0.2 to 1.0 mV s-1 were applied as follows (illustrated in Figure 6a). The voltammetric sweep rate dependence can be utilized to determine the capacitive contribution to the current response

33, 39

. The capacitive effect of the battery system is calculated according to

Eq. (3) and (4) (where i is the peak current, ν is the scan rate and a and b are adjustment parameters.): i=aνb

(Eq. 3)

log i = b × log ν + log a

(Eq. 4)

The b value, which is a critical parameter for inferring the electrochemical process, which can be estimated from the fitted line of log ν and log i. When the b value comes close to 1, surface pseudocapacitive sodium storage dominates on the NiS2-GNS electrode, while if the b value approaches to 0.5, conversion reactions play the main role. Figure 6b shows log ν – log i plots of the cathodic process at ~0.95 V and the anodic process at ~1.70 V. The b value of the cathodic process is 0.49, indicating that the conversion reaction dominated at around 0.95 V when discharging, while the b value of anodic process is 0.71, demonstrating that it might be a mixed process with both capacitive desorption and conversion when charging to around 1.70 V. Given the detailed difference about those two reaction mechanisms in terms of the current contribution, as highlighted by Yan et al.

40

, we presume the mixed process can be

described by the following equation: Ip = C1ν + C2ν1/2

(Eq. 5)


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In Eq. (5), Ip represents current density, ν is the scan rate; C1ν and C2ν1/2 correspond to the current contributions form surface adsorption/desorption effects (Is) and bulk conversion effects (Ib), respectively. For simplifying computation procedure, Eq. (5) can be rewritten as Ip/ν1/2 = C1ν1/2 + C2

(Eq. 6)

Calculated and fitted data are illustrated in Figure S4. For the oxidation peak at 1.70 V, C1 = (0.430 ± 0.005) and C2 = (0.411 ± 0.002). According to these results, specific values of Is and Ib at different scan rates can be obtained, which can be used for assessing relative contribution of surface adsorption/desorption and bulk conversion at around 1.70 V. Figure 6c demonstrates the calculated surface desorption current and bulk conversion current densities as a function of scan rate. At the scan rate of 0.2 mV s-1, Is is half of Ib, indicating that bulk conversion reaction dominated at the low scan rate, along with noticeable surface desorption. As the scan rates increase, both Is and Ib show an upward trend, while Is grows faster than Ib, finally going beyond Ib at the scan rate of 1 mV s-1. The fact can be concluded from these results that both surface desorption and bulk conversion play important roles during charge process at around 1.70 V. In terms of discharge process at around 0.95 V, C1 = (0.108 ± 0.003), C2 = (0.506 ± 0.001), and the corresponding Is (Ib) – ν plots are illustrated in Figure 6d. Ib is much higher than Is at all the scan rates while Is has a bigger increment speed than Ib during scan rate increasing. Even so, Ib stays ahead of Is by 500% when the scan rate reaches a high value of 1.0 mV s-1, showing that bulk conversion is the predominant reaction at around 0.95 V.



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These

results

demonstrate

that

the

capacitive

contributions

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of

surface

absorption/desorption and bulk conversion process vary at different stages of the charge-discharge process, confirming that it is a mixed process on the NiS2-GNS electrode in SIBs.

Figure 7. EIS for SIBs with NiS2-GNS anode and pristine NiS2 anode. Electrochemical impedance spectroscopy measurements were further carried out to compare the intrinsic electrochemical behaviors between NiS2-GNS and pristine NiS2 as the anode electrodes in SIBs before cycling. As seen in Figure 7, both curves consist of a semicircle in high-frequency region, which represents the charge-transfer resistance (Rct) at the interface between the electrode and electrolyte, and a slash in the low-frequency region, indicating the ion diffusion in the bulk electrode. The inset equivalent circuit model (Figure 7) was used to simulate the EIS spectra with the corresponding fitted data elucidated in Table S2. The values of the SEI film resistance Rf of pristine NiS2 and NiS2-GNS are 270.9 and 165.2 Ω, respectively. The Rct values of pristine NiS2 and NiS2-GNS are 357.1 and 182.6 Ω. The results demonstrate that after incorporated with graphene sheets, the charge transfer resistance and the SEI film resistance of the hybrid composite show an obvious decrease compared to that of


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pristine NiS2 electrode, which confirms again that graphene sheets can be greatly helpful to improve the electrical conductivity of the anode materials.

CONCLUSIONS In summary, NiS2-GNS composites have been efficiently synthesized via a mild one-step hydrothermal method and introduced as the anode material in SIBs here for the first time. NiS2-GNS composites showed superior cycle performance and rate capability compared to pristine NiS2 electrode. The enhanced electrochemical performance of NiS2-GNS can be attributed to the synergistic effect between NiS2 nanoparticles and graphene nanosheets; and it is suggested that the combination of graphene nanosheets might be effective to relieve the aggregation of NiS2 nanoparticles and enhance the electrical conductivity of the composite.

AUTHOR INFORMATION Corresponding Author. *E-mail: [email protected]. Tel.: +8653266781688. *E-mail: [email protected]. Tel.: +8653280662746. *E-mail: [email protected]. Author Contributions †

T. W. and P. H. contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21473228), China Postdoctoral Science Foundation (2014M561976) and Shandong Provincial Natural Science Foundation (BS2015CL014).



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Supporting Information Additional Figure S1-S4 and Table S1-S2. TG curve of NiS2-GNS composites (heated in air with a rate of 10 °C min−1). SEM images of the NiS2-GNS electrode before and after 100 cycles. XRD pattern of the NiS2-GNS electrode (primary state and after 100 cycles). The calculated C1 and C2 for two samples using Eq. (2) that correspond to the slope and the y-axis intercept point, respectively. 2θ, FWHM and calculated crystallite sizes of the NiS2-GNS and pristine NiS2. Fitted data of the Nyquist plots of the pristine NiS2 and the NiS2-GNS electrode. REFERENCES (1). Kim, Y.; Ha, K. H.; Oh, S. M.; Lee, K. T., High-capacity Anode Materials for Sodium-Ion Batteries. Chem. - Eur. J. 2014, 20 (38), 11980-92. (2). Yoshida, H.; Yabuuchi, N.; Kubota, K.; Ikeuchi, I.; Garsuch, A.; Schulz-Dobrick, M.; Komaba, S., P2-type Na2/3Ni1/3Mn2/3−xTixO2 as A New Positive Electrode for Higher Energy Na-Ion Batteries. Chem. Commun. 2014, 50 (28), 3677-3680. (3). Billaud, J.; Singh, G.; Armstrong, A. R.; Gonzalo, E.; Roddatis, V.; Armand, M.; Rojo, T.; Bruce, P. G., Na0.67Mn1−xMgxO2 (0≤x≤0.2): A High Capacity Cathode for Sodium-Ion Batteries. Energy Environ. Sci. 2014, 7 (4), 1387-1391. (4). Kim, H.; Park, I.; Seo, D.-H.; Lee, S.; Kim, S.-W.; Kwon, W. J.; Park, Y.-U.; Kim, C. S.; Jeon, S.; Kang, K., New Iron-based Mixed-Polyanion Cathodes for Lithium and Sodium Rechargeable Batteries: Combined First Principles Calculations and Experimental Study. J. Am. Chem. Soc. 2012, 134 (25), 10369-10372. (5). Barpanda, P.; Ye, T.; Nishimura, S.-i.; Chung, S.-C.; Yamada, Y.; Okubo, M.; Zhou, H.; Yamada, A., Sodium Iron Pyrophosphate: A Novel 3.0 V Iron-based Cathode for Sodium-Ion Batteries. Electrochem. Commun. 2012, 24, 116-119. (6). Bommier, C.; Luo, W.; Gao, W.-Y.; Greaney, A.; Ma, S.; Ji, X., Predicting Capacity of Hard Carbon Anodes in Sodium-Ion Batteries Using Porosity Measurements. Carbon 2014, 76, 165-174. (7). Bresser, D.; Mueller, F.; Buchholz, D.; Paillard, E.; Passerini, S., Embedding Tin Nanoparticles in Micron-Sized Disordered Carbon for Lithium-and Sodium-Ion Anodes. Electrochim. Acta 2014, 128, 163-171. (8). Kim, S. W.; Seo, D. H.; Ma, X.; Ceder, G.; Kang, K., Electrode Materials for Rechargeable


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